Jökull - 01.01.2015, Blaðsíða 22

Sigmarsson and Halldórsson
Figure 2. Strontium isotope ratio of Recent lavas from the Bárðarbunga-Veiðivötn and Askja volcanic systems
demonstrating the connection of Holu-, Krepputungu- and Fjallsendahraun (Frambruni) lavas to the Bárðar-
bunga magma system. External analytical errors are represented by the width of the columns. Same symbol
as for the Þjórsár lava is used for two old Veiðivötn lavas. "Uncertain" refers to lavas that have not been cor-
related to a given volcanic system (see text for further discussion). – Samsætuhlutföll Sr í nútíma basalti frá
eldstöðvakerfum Bárðarbungu-Veiðivatna og Öskju sýna að Holuhraun og Krepputunguhraun eiga rætur sínar
að rekja til kvikukerfis Bárðarbungu. Sömu tákn eru notuð fyrir tvö af elstu hraunum Veiðivatnareinarinnar og
Þjórsárhraun sem skera sig úr mengi annara Veiðivatnabasalta. Sér tákn er notað fyrir hraun sem ekki hafa
verið rakin til ákveðinna eldstöðvakerfa.
Older results from Hémond et al. (1993) agree
perfectly with the results shown in Figures 2 and
3 after readjustment to the same standard values.
The only isotope ratios from Bárðarbunga that dis-
agree with ours are those attributed to the Tungnaár-
hraun lava by Kempton et al. (2000) with 87Sr/86Sr
equal to 0.70327. Our analysis of the Búrfellshraun
(Tungnaárhraun) gave 0.70312±1, confirmed by the
results of Manning and Thirlwall (2014) who obtained
87Sr/86Sr equal to 0.70311±1. Earlier less precise
Nd isotope ratios, such as those of Sigmarsson et al.
(2000), are excluded from this compilation. Three
lavas from the Veiðivötn fissure swarm have signifi-
cantly higher 87Sr/86Sr than other lavas from the same
volcanic system (Figure 2). These are amongst the
oldest Veiðivötn lavas, namely Þjórsárhraun, Jökul-
heimahraun and Skeifuhraun (or Fellsendahraun; see
maps in Vilmundardóttir et al., 2000 and Jakobsson,
1979). Notably, these basalts were produced in the
early Holocene after a large isostatic rebound fol-
lowed the rapid ice melting, which possibly affected
the melting regime at the origin of these basalts. In-
deed, a detailed study of Sr isotopes in plagioclase
crystals and the groundmass of the Þjórsárhraun lava
revealed lower 87Sr/86Sr for the crystals, which was
proposed to reflect mixing between mantle melts and
earlier-formed mafic intrusions, highligths the com-
plex origin of this large lava unit (Halldórsson et al.,
2008).
Based on the filtering criteria adopted, the range,
median, mean and standard deviation for both Sr
and Nd isotope ratios of Bárðarbunga-Veiðivötn lavas
22 JÖKULL No. 65, 2015